In many cases, position devices and angular measuring devices are used for automatic positioning and measuring in machine tools and coordinate measuring devices. Rotary angle sensors, for example, are used in motor vehicles for determining the absolute angular position of the steering wheel and therefore also are called steering angle sensors. The steering angle is required in motor vehicles, in order to be able to apply this value, for example, to the system for controlling the dynamics of vehicle movement. Such a system for controlling the dynamics of vehicle movement, in addition to the aforementioned steering angle values, contains other measuring data, for example, the number of rotations of the wheel or the vehicle's rotation around its vertical axis. Both the absolute steering angle and the speed of steering are required, so that these values, together with the other sensed data, can be evaluated by the system for controlling the dynamics of vehicle movement and be converted for controlling the actuators, for example, the brakes and/or the engine management.
An optoelectronic steering angle sensor, for example, is known from DE 40 22 837 A 1. The steering angle sensor described in this document consists of two parallel disposed elements, a light source and a line sensor, as well as a code disk placed between the light source and the line sensor, which disk is fixed to the steering wheel shaft. A CCD sensor line serves as a line sensor. With this code disk, a light slit formed as an archimedean spiral, which extends over 360°, is provided as coding. Lighting of the corresponding transducing elements of the line sensor at a specific steering angle can provide information on the actual steering angle.
Other optoelectric steering angle sensors are known which, instead of the above-described analog coding, comprise a digital coding, for example, a so-called gray code. The resolution of such a digital code is contingent upon the length of the smallest structure. Code disks, the digital coding of which is able to realize a resolution of approx. 0.7°, are produced at a reasonable cost and are used in the motor vehicle segment. As the demands on the steering sensor systems increase, it is necessary to provide a higher resolution capacity. An increase in the resolution capacity by improving the digital code would be feasible, but is too costly, because of the fineness of the necessary structures.
DE 196 04 502 A1 describes an optoelectronic read head for reading digital codings, in which both the light source and an optical sensor are disposed on the same side of a code disk. A gray code serves as a code, as well as an analog track with wedge-shaped signal surfaces. The analog track, which serves to increase the resolution capacity of the device, has a saw-toothed shape and contains irregularities in which a defined signal assignment is not possible. The irregularities are linked by slanted signal surfaces, in which it is provided that only the center section of the slanted signal surfaces is evaluated.
The code length of a code segment of the analog track, which extends from the first irregularity to the next irregularity by extending the analog track, corresponds to the length of the spacing of the digital gray code. However, because only the center section of such an analog code segment is used to avoid an evaluation of the irregularities, the effective code segment length corresponds to the analog track of the signal element length of the gray code. In order to still achieve a continuous readout of the analog signal surfaces with the object of DE 196 04 502 A1, the analog signal surfaces are scanned with two streaks of light. This, however, makes considerable demands on the hardware used therein, such as mirrors, etc., resulting in inaccuracies during the evaluation when switching from one streak of light to another. Consequently, a continuous evaluation of the analog signal surfaces with the coding known from DE 196 04 502 A1 cannot be performed without errors.
From DE 40 14 479 A1 another optoelectric position or angular measuring device is known, whose code disk comprises a digital code as well as an analog track. The analog track is formed of lens-shaped, individual, not interconnected code track segments. The device described in DE 40 14 479 A1 operates like a light barrier, in which a detector is allocated to each digital code track. The analog code track comprises two adjacent detectors. A sensor array, for example, a line sensor, is not provided with the object of this document. Even if the analog code track within a specific angular range is able to achieve a higher resolution by means of the lens-shaped code track segments, an even resolution over the entire extension of the coding is not yielded. The resolution increases in the transitions, if one or both detectors of the analog track are exposed. A change cannot be detected in the longer sections, for example, within a lens-shaped code track segment or between two code track segments.
In addition, a further reference track is located on the code disk, which is light-permeable throughout and which has its sole purpose to take into consideration and to eliminate differences in light during the evaluation with illumination of detectors which are not to be traced to a movement of the code disk.
The problem with this type of optoelectronic position or angular measuring device is the mechanical play of the moveable code disk relative to the fixed detectors, which, above all, with the object of DE 196 04 502 A1 is significant, because of the necessary switchover of the streak of light. The correction of these types of mechanical tolerances is not taken into consideration with any of the cited documents. In the object of document DE 40 14 479 A1, this is not absolutely necessary due to the installed detector photodiodes.